There seems to be a serious continuity problem here. In Chapter 4, Rinker is dismayed to discover that his "Trail Pup" won't work for the journey. As a backup plan, he's sure to select a mule that he can actually ride. Mere pages later, they're putting a sign on the back of the Trail Pup, and then in subsequent chapters, it's clear they've brought it along on the journey. What? Did I miss something?
Lisa I thought it was confusing, too. The way I understood it was that the Trail Pup was okay for being pulled behind the wagon, but it wouldn't work being hitched to a mule alone. Hence, they could still take it for storage, but needed the mule to ride in case of an emergency.
Amy The way I understood it, the issue with the trail pup was as a "backup plan" of being able to use the mules to haul it separately for help if the wagon was disabled. Because of the weight balance of the pup (too heavy on the tongue), this was impossible. So, the backup plan became riding the mule for help.